Lights New Car Doubt

[A17APP0002 (Akshay Bhatia)]

I did not understand the part that how B= a * (p-1) + b and then how b= B%(p-1) ?

[abhishek_trigunait]

Do tell me if still unclear!

[tejasddongare (Tejas Dongare)]

sir how are we getting B=y%(m-1) in last step ???